1. Field of the Invention
This invention relates to electronic switching circuits, and in particular to a solid state switching circuit for use in the tip and ring circuit of a rotary dial telephone pulsing system as a functional replacement for a single pole, double throw electromechanical pulsing relay having normally open and normally closed contacts.
2. Description of the Prior Art
A relay is an electric switching device comprising one or more contacts which open or close circuits. In some relays, the switching action is brought about by an electromagnet which closes or opens contacts by means of a movable armature which it attracts or releases.
The switching action of an electromechanical relay is characterized by several factors including mechanical bounce and electrical waveform distortion which occurs in response to the bounce, welding of relay contacts in response to arcing and high current flow, pitting of relay contacts which increases circuit resistance, relatively slow response time, relatively low reliability which gradually diminishes with age, and latching current sensitivity. Moreover, the solenoid of the electromechanical relay introduces a high inductive load within the operating circuit.
The foregoing operating characteristics limit, in some way, the use of the electromechanical relay for various applications.
Telephone exchange equipment is an important application area in which electromechanical relays have played an essential role. When a subscriber dials a number, the circuit to the telephone exchange is interrupted as many times as corresponds to that number. Each digit of the dial controls a selection stage comprising an electromechanical multiple contact stepping switch. The rotary dial pulses are coupled to a stepping relay and a timing relay by means of a single pole, double throw pulsing relay having normally closed and normally open switch contacts. The solenoid of the pulsing relay is energized by the flow of current in the subscriber circuit.
It will be appreciated that the latching current sensitivity of the pulsing relay will be affected by the condition of the telephone equipment, including the subscriber circuit, the telephone transmission lines and power supply regulation. As the equipment grows older, the latching current sensitivity, together with the usual problems of bounce, contact welding, contact pitting, response time and inductive loading, combine to significantly reduce the operating reliability and effectiveness of the rotary dial telephone system.